LTO inliner -- sensitivity to increasing register pressure

[Aaron Sawdey <acsawdey at linux dot vnet dot ibm dot com>]

Honza,
  Seeing your recent patches relating to inliner heuristics for LTO, I 
thought I should mention some related work I'm doing.

By way of introduction, I've recently joined the IBM LTC's PPC Toolchain 
team, working on gcc performance.

We have not generally seen good results using LTO on IBM power processors 
and one of the problems seems to be excessive inlining that results in the 
generation of excessive spill code. So, I have set out to tackle this by 
doing some analysis at the time of the inliner pass to compute something 
analogous to register pressure, which is then used to shut down inlining of 
routines that have a lot of pressure.

The analysis is basically a liveness analysis on the SSA names per basic 
block and looking for the maximum number live in any block. I've been using 
"liveness pressure" as a shorthand name for this.

This can then be used in two ways.
1) want_inline_function_to_all_callers_p at present always says to inline 
things that have only one call site without regard to size or what this may 
do to the register allocator downstream. In particular, BZ2_decompress in 
bzip2 gets inlined and this causes the pressure reported downstream for the 
int register class to increase 10x. Looking at some combination of pressure 
in caller/callee may help avoid this kind of situation.
2) I also want to experiment with adding the liveness pressure in the 
callee into the badness calculation in edge_badness used by 
inline_small_functions. The idea here is to try to inline functions that 
are less likely to cause register allocator difficulty downstream first.

I am just at the point of getting a prototype working, I will get a patch 
you could take a look at posted next week. In the meantime, do you have any 
comments or feedback?

Thanks,
   Aaron

--
Aaron Sawdey, Ph.D.  acsawdey@linux.vnet.ibm.com
050-2/C113  (507) 253-7520 home: 507/263-0782
IBM Linux Technology Center - PPC Toolchain